2. _The physical strength of workers differs according to age, individual, race, and sex._ Differences due to age are the most obvious. The child, at first weak, grows toward his maximum of physical strength, which he attains before his fullest intellectual capacity. The period of maximum physical working power lasts fifteen to twenty-five years according to the individual, and then gradually declines as the old worker approaches again the inefficiency of the child. Mental efficiency develops more slowly and longer, the highest qualities of judgment and wisdom being the fruits only of a life rich in experiences. Families and strains of stock differ notably in physical and mental powers; one excels in stature, another in development of muscle. The differences within families are inexplicable, sometimes one brother excelling in one thing, the other in another. The physically perfect man is a rare product. Among three thousand students are but two score endowed with the remarkable combination of lungs, heart, muscle, nerve, and character, that makes possible the finest athletes. The national and racial differences in working power, even in the simplest tasks, are marked but difficult to explain, as so many influences of customs, habits of life, and varieties of diet modify the result. We cannot tell how much of the Englishman's great superiority over the East Indiaman is due to individual, native differences of mind and body, how much to the social environment in which they have lived. Certainly, though, the difference is not mainly one in size; in the Chinese War the little brown men of Japan outmarched all the others. Certainly fiber counts for more than bulk, and character for more than muscle. [Sidenote: Comparative strength of men and women]